[XHR] Setting responseType after calling send() should be allowed.

XHR.responseType can be changed if the readyState is not LOADING or DONE. This
CL makes it able to change the responseType even if the response type was
set to "blob" when send() was called.

BUG=412705
R=tyoshino

Review URL: https://codereview.chromium.org/607393002

git-svn-id: svn://svn.chromium.org/blink/trunk@183173 bbb929c8-8fbe-4397-9dbb-9b2b20218538